South Dakota Sen. Tim Johnson won't seek re-election

South Dakota Sen. Tim Johnson will not seek re-election in 2014, NBC News has confirmed. He is expected to make a formal announcement of his retirement Tuesday.

Johnson’s decision opens an opportunity for Republicans to take over the seat in a state that favored Mitt Romney over Barack Obama by almost 20 points.

His departure is no great surprise to operatives on both sides of the aisle. Johnson has been recovering from a brain hemorrhage in 2006 that left him partially paralyzed. (He did, however, run successfully for re-election in 2008.)